This invention relates to a method of purifying t-butylstyrene and, more particularly, to a process for preparing t-butylstyrene by the controlled alkylation of ethylbenzene with isobutylene in the presence of cooled concentrated sulfuric acid to form t-butylethylbenzene containing at least about 95 wt. % of the p-isomer, oxidatively dehydrogenating the t-butylethylbenzene in the vapor phase over an alkaline pyrophosphate catalyst, and purifying the resulting mixture rich in t-butylstyrene from unreacted t-butylethylbenzene and small amounts of alkenyl-substituted styrene impurities by a combination of a vacuum fractionation and vacuum evaporation in the presence of a polymerization inhibitor, and treatment with a carbonaceous adsorbent.
Styrene and alkyl-substituted styrene production has a long history. In the past such styrenes have been made principally by thermally or catalytically dehydrogenating, for example with steam or oxygen, the corresponding ethylbenzene or alkylethylbenzene. When making an alkyl-substituted styrene from an alkyl-substituted ethylbenzene, a problem arises in regard to the alkyl substituent and care must be exercised to dehydrogenate the ethyl group with a minimum amount of cracking or dehydrogenation of the alkyl substituent. As the number of carbon atoms in the alkyl group becomes greater, for example, dehydrogenating t-butylethylbenzene, this problem becomes more severe. A number of catalysts have been suggested for the catalytic dehydrogenation of styrene and its alkyl derivatives including alkaline earth pyrophosphates as set out in U.S. Pat. No. 3,917,732.
The alkyl-substituted ethylbenzene starting material for the manufacture of the corresponding styrene, such as t-butylethylbenzene, has commonly been made by alkylating ethylbenzene. The alkylation process can produce ring isomers with the para isomer being the most desired. Different alkylation catalysts, which produce different isomer ratios, conversions, and selectivities, such as sulfuric acid, boron trifluoride, aluminum chloride, and some mixtures thereof, have been suggested for the alkylation process. Typical is the description set out in U.S. Pat. No. 3,631,213.
t-Butylstyrene is a compound which has many uses, e.g., as a chemical intermediate and as a monomer or comonomer in the production of polymeric materials. t-Butylstyrene has replaced styrene in some applications because of the desirable physical and chemical properties that result from such a substitution. In addition, there are processes in which styrene is not suitable but where t-butylstyrene functions well.
Because t-butylstyrene belongs to the same family as styrene, there are similarities in the chemistry of its preparation. There are differences also; for example, when t-butylethylbenzene is dehydrogenated in the vapor phase, particularly at higher temperature which can optimize conversion and selectivity, small amounts of dialkenylbenzene impurities are produced. Since one of the common properties of styrene and an alkenyl-substituted styrene is the tendency to polymerize whenever they are activated by chemicals or by heat, both the dialkenyl impurities and the styrene can polymerize during manufacture, purification, and polymerization.
Although some of the fractionation techniques useful in purifying styrene can be used to purify t-butylstyrene, the boiling point of t-butylstyrene is about 70.degree. C. higher than that of styrene and the tendency for t-butylstyrene to polymerize during fractionation is much greater than that of styrene. Thus, none of the present commercial processes for purifying styrene are acceptable for the purification of t-butylstyrene.
As written above, some of the differences between styrene and t-butylstyrene derive from compounds of the dialkenylbenzene family which are present in larger amounts in t-butylstyrene than in styrene. These crosslinking compounds can polymerize to give a type of polymer that interferes with the operation of the purification process and equipment. The crosslinked polymer has a tendency to collect in the equipment and to resist attempts to dissolve it, and even small amounts can represent a severe problem in the commercial production, purification, and polymerization of t-butylstyrene.
Some other differences from styrene that arise during separation of t-butylstyrene from impurities into a pure product derive from the presence of the ring isomers of t-butylstyrene and t-butylethylbenzene. Both meta and para isomers are found in commercial t-butylethylbenzene and in the t-butylstyrene formed from it. The m-(t-butyl)styrene has a boiling point that is intermediate between the boiling points of p-(t-butyl)ethylbenzene and p-(t-butyl)styrene and is likely to be found in both the top and bottom tower cuts if a conventional distillation column is used for separating t-butylethylbenzene and t-butylstyrene. Thus, the process for making t-butylethylbenzene should be chosen to optimize production of the para isomer if p-(t-butyl)styrene is the desired product. Removing the other isomers in the purification process for t-butylstyrene can be both inconvenient and expensive.
In application Ser. No. 646,266, filed Aug. 31, 1984, an extractive distillation process is disclosed for the separation of t-butylstyrene from an oxidative dehydrogenation reactor effluent containing t-butylethylbenzene and t-butylstyrene using anhydrous sulfolane as solvent. This process is said to work well but the use of reduced pressure and elevated temperature contribute considerably to the costs of the overall process. Trials with anhydrous sulfolane as solvent in liquid/liquid extraction of oxidative dehydrogenation reactor effluent indicate that pure t-butylstyrene could not be produced because anhydrous sulfolane and t-butylstyrene are completely miscible.
Preparing t-butylstyrene by oxidatively dehydrogenating t-butylethylbenzene in the vapor phase is a one-step process unlike the multistep process developed in the U.S. by Dow in the 1970's. However, oxidatively dehydrogenating t-butylethylbenzene produces several by-products (dialkenyl compounds) which are difficult but necessary to separate from t-butylstyrene. Also, the dehydrogenation process requires starting with a t-butylethylbenzene having a very high para/meta isomer ratio if the p-isomer of the alkylstyrene is desired. If the by-products could be easily separated and a high para/meta isomer t-butylethylbenzene feed used, the one-step process could considerably more economical.
Dialkenyl compounds even in concentrations as low as 200 ppm are effective crosslinking agents that cause formation of insoluble gel during purification and polymerization of the monomer resulting in an inferior polymer. In order to use the potentially more commercial one-step oxidative dehydrogenation, an effective commercial method to rid the dehydrogenation product from dialkenyl compounds as well as unreacted t-butylethylbenzene needs to be found, and, if p-(t-butyl)styrene is the desired product, t-butylethylbenzene with a high para/meta isomer ratio must be used as the starting material in the dehydrogenation step.
Now it has been found that the vapor phase dehydrogenation of t-butylethylbenzene to t-butylstyrene can be employed and run at a temperature giving good conversion and selectivity if the dehydrogenation is run oxidatively on a very high para to meta isomer ratio starting material, and, if a combination of vacuum fractionation, vacuum evaporation, and treatment with a carbonaceous adsorbent is used in the t-butylstyrene purification process to remove unreacted t-butylethylbenzene and such difficult-to-separate dialkenyl benzene compounds as isopropenylstyrene and sec-butenylstyrene.